I would think the issue would be that the output mode of the POD will default to STACK PWR AMP when connected to the DT25, which would make the output from the L2t sound poor. Line 6 advise to connect the DT25 first in the L6 link chain - I'm not sure if connecting the L2t first would force STUDIO/DIRECT mode. To me, this would make more sense as you could assign the DT25 to one of the amp models and select "no cab" to inhibit the cab and mic simulation.

There isn't really much guidance from Line 6 about how to use these two amps together in a guitar rig, although it is inferred in the POD HD Pro Advanced Guide page 2-17:

"Note: It is necessary to always place the StageSource speaker(s) last in the L6 LINK signal chain (e.g. - POD HD Pro > DT50 > StageSource L3). For more about using the L6 LINK connection, DT Series amps & StageSource speakers, please see the additional documentation available at http://line6.com/support/manuals/."

And for the life of me, I can't find anything more on this topic at the URL quoted in the manual. The L6 link connectivity guide only covers DT amps.

Well, I've just taken delivery of an L3t today, so I've given this a go...

You've got it pretty much spot on. The key is having the L2/L3 speaker mode set to electric guitar and not reference PA. That accounts for the POD output mode being forced to STACK PWR AMP when connected to the DT via L6 link. I put pan pedal after the mixer to pan between the DT and L3 and although they sound slightly different (which might be expected as I was using my own cab with the DT), both sounded really good. I just used a JCM 800 model with everything set to 12 o'clock on the pre- and full models. I can see myself spending hours setting up some nice dual and stereo patches now